1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal display device and method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing touch and gravity degradation in the LCD device.
2. Discussion of the Related Art
Much effort has been made to research and develop various kinds of flat panel display devices as the demands for display device rise due to the ongoing development of the information age. The various kinds of flat panel display devices are LCD (liquid crystal display), PDP (plasma display panel), ELD (electroluminescent display), VFD (vacuum fluorescent display) and the like. Some of these flat panel display devices are already in use in various kinds of instruments.
The LCD has the advantageous of light weight, thin profile and low power consumption. Thus, the LCD has been replacing the CRT (cathode ray tube) and is being used as a mobile image display device. For example, the LCD is used as a notebook computer monitor for a mobile usage. Further, the LCD can be used in less mobile applications, such as a TV monitor or a computer monitor. In order for the LCD to be a general image display device for various fields, the LCD not only has to maintain the features of the light weight, thin profile and low power consumption but also needs to be capable of implementing high-grade image having high definition and high luminance on a large-sized display panel.
In general, a liquid crystal display device includes a first substrate, a second substrate bonded to the first substrate with a predetermined gap in-between the first and second substrates, and a liquid crystal layer injected between the first and second substrates. A plurality of spaced apart gate lines crossing a plurality of spaced apart at data lines define pixel areas on the first substrate. A pixel electrode is provided in each of the pixel areas. A thin film transistor is also provide in each of the pixel areas adjacent to where a data line and a gate line of the pixel area cross each other. The thin film transistor of a pixel area provides a data signal from the corresponding data line of the pixel area to the corresponding pixel electrode of the pixel area in response to a signal applied to the corresponding gate line of the pixel area.
A black matrix layer is formed on the second substrate, except for the pixel areas, to block light. Red (R), green (G) and blue (B) color filter layers are formed within areas of the black matrix corresponding to each of the pixel areas to display colors. A common electrode is formed on the color filter layers for forming an electric field with the pixel electrodes to implement an image. More particularly, liquid crystal molecules of the liquid crystal layer between the first and second substrates are aligned by an electric field generated between the pixel electrode and the common electrode. By adjusting a quantity of light transmitted through the liquid crystal layer according to the degree of alignment of the liquid crystal layer, an image can be displayed.
The above-configured liquid crystal display device is called a TN (twisted nematic) mode liquid crystal display device. The TN mode liquid crystal display device has the disadvantage of a narrow viewing angle. To overcome this disadvantage of the TN mode liquid crystal display device, an in-plane switching (IPS) mode liquid crystal display device has been developed. In the IPS mode liquid crystal display device, a pixel electrode and a common electrode are formed parallel to each other in a pixel area of a first substrate with a predetermined distance with each other. A transverse electric field is generated between the pixel and common electrodes to align the molecules of a liquid crystal layer.
In both the TN mode and IPS mode, spacers are formed between the first and second substrates of the above-configured liquid crystal display device to sustain a predetermined gap for accommodating the liquid crystal layer. The spacers can be classified as either ball spacers or column spacers, depending on the shape of the spacers. The ball spacers are spherically shaped and are scattered on the first or second substrate. After the first and second substrates have been bonded to each other, the ball spacers are relatively free to move. And, each of the ball spacers has a relatively small contact area with the first or second substrate.
On the other hand, the column spacers are formed during an array fabrication process of the first or second substrate. Each of the column spacers has a pillar shape having a predetermined height that is affixed to a predetermined substrate. Hence, a contact area of the column spacer with the first or second substrate is relatively greater than the contact area of the ball spacer with the first or second substrate. A related art liquid crystal display device having a column spacer according to a related art is explained with reference to the attached drawings as follows.
FIG. 1 is a cross-sectional diagram of a related art liquid crystal display device having a column spacer. Referring to FIG. 1, a liquid crystal display device having a column spacer includes a first substrate 30, a second substrate 40 facing the first substrate 30, a column spacer 20 provided between the first and second substrates 30 and 40, and a liquid crystal layer (not shown in the drawing) provided between the first and second substrates 30 and 40. A gate line 31 and a data line (not shown in the drawing) are arranged to cross each other on the first substrate 30 to define a pixel area. A thin film transistor (TFT) is formed adjacent to the crossing between the gate line 31 and the data line. A pixel electrode (not shown in the drawing) is formed in the pixel area.
Excluding the pixel area, a black matrix layer 41 is formed on the second substrate 40. A stripe type color filter layer 42 is formed on the black matrix layer corresponding to the pixel area on a vertical line parallel to the data line. A common electrode or an overcoat layer 43 is formed over the second substrate 40. A gate insulating layer 36 is formed over the first substrate 30, including the gate line 31, and a passivation layer 37 is formed on the gate insulating layer 36. The column spacer 20 is formed at a predetermined position over the gate line 31.
FIG. 2A and FIG. 2B are a plan view and a cross-sectional diagram of a related art liquid crystal display device having a column spacer, respectively. Referring to FIG. 2A and FIG. 2B, a stain is formed on a part of a liquid crystal panel 10 of the above-explained liquid crystal display device having the column spacer if a surface of the part of the liquid crystal panel 10 is touched by a hand or other external object. This stain is called a touch stain, which is generated on the liquid crystal panel 10 while the panel is being touch. If the stain remains on the panel 10 after being touched, it is called a touch degradation.
A touch stain is caused by a shift between the first and second substrates 1 and 2 due to a touch. The touch degradation is attributed to a large contact area of the column spacer 20 having considerable friction between the column spacer 20 and the first substrate 1, as compared to the friction that would be generated between the ball spacer structure and the first substrate in which the ball spacer structure has less contact area with the first substrate 1. Because of the friction between the column spacer 20 having a column shape and the first substrate 1, as shown in FIG. 2B, it takes a considerably long time for the stained panel to return to an original state after a shift between the first and second substrates 1 and 2 is caused by a touch. Hence, the stain remains until the shift between the first and second substrates 1 and 2 recovers back or is undone.
The related art liquid crystal display device with the column spacer has the following problems. First, the contact area between the column spacer and the confronting substrate is relatively large such that the static friction is large and if the LCD panel has substrate shifting due to a touch, it takes a considerably long time for a panel to recover. Hence, the touch degradation is observed during the recovery of the panel. Second, if the LCD panel having the column spacer is held upright and is placed in an environment at a high temperature, the liquid crystals go through thermal expansion such that a cell gap is extended more than the height of the column spacer, which enables liquid crystal molecules to flow downward and make a lower end of the LCD panel bulge, which causes an opaqueness in the LCD panel.